Välj var radnummer =radnummer

Slå in den i en subquery

SELECT c.*
FROM
   (
      SELECT @row := @row + 1 AS row, pi.*
      FROM ci_patient_info pi
   ) c
where c.row = 2

UPPDATERING 1

kanske din underfråga ska se ut så här,

  SELECT @row := @row + 1 AS row, pi.*
  FROM ci_patient_info pi, (SELECT @row:=0) x